The method according to the invention is intended for use especially in trunked networks, which are typically networks of companies or authorities wherein all channels are used by one or several user organizations. In addition to their own subscriber numbers, the subscribers in these networks also have group numbers, which indicate the group call group or subscriber group the subscriber belongs to, so that calls intended for subscribers of a particular group can be forwarded to the subscribers in question.
The invention is applicable in both mobile communication systems with a digital radio path and systems with an analog radio path. Analog mobile communication systems are described for example in the following publications by the British Department of Trade and Industry: "MPT 1327, January 1988, Revised and reprinted November 1991, A Signalling Standard for Trunked Private Land Mobile Radio Systems, Radio-communications Agency" and "MPT 1343, January 1988, Revised and reprinted September 1991, Performance Specification, Radiocommunications Agency".
A group call is one of the central functions of a private mobile radio communication system. A group call is used, for example, in all kinds of operations with several participants, especially when an entire group must be continuously kept up to date in regard to certain events. A group call is a conference call wherein all participants can speak in their turn and also hear each other. In group calls, the entire group is called by using a single call number. An individual radio unit, such as a mobile station or a radio phone, i.e. a subscriber station, may belong to several groups that are programmed into the radio unit. The programming may be fixed, but the user of the mobile station may also change it. The system stores a file of the base stations associated with each group number. A group call may cover one, several or all base stations within the area of a mobile exchange, or several mobile exchanges. When a group call is being established, a traffic channel is reserved from all base stations belonging to the group, and each of these base stations transmits a group call message comprising a group number and information about the reserved traffic channel. If the mobile station identifies the group number contained in the group call message, it moves to the traffic channel indicated in the group call message. Therefore it is in principle always possible to engage a mobile station in a group call if the mobile station is within the area of operation predetermined for the group and if it is not already engaged in another group call.
In the following, the operation of a mobile communication system employing group calls is illustrated in a normal situation by means of FIG. 1. In a normal situation, mobile stations must be able to communicate in a group call group comprising all radio phones and the dispatcher of the dispatching area (FIG. 1). For example, radio phones 111, 112 and 113 and dispatcher A in AREA 1 belong to group call group R1, and when they call each other they use the identifier of the group call group R1, i.e. they dial the number R1. The other dispatching areas AREA 2 and AREA 3 operate in the same manner with their own call group identifiers R2 and R3.
FIG. 2 shows an example of the operation of a mobile communication system during an expanded group call, i.e. for example during night dispatching. Not all dispatchers are needed during such quiet periods of the user organization, for example at night, wherefore dispatching areas can be combined. FIG. 2 illustrates the combining of dispatching areas so that the dispatchers A and B have stopped operating and the group calls R1 and R2 previously under their control, as well as the corresponding geographical areas AREA 1, AREA 2,have been placed under the control of dispatcher C. In this application, night dispatching and an expanded group call hereinafter refer to a situation where dispatcher groups have been combined.
In a prior art system, night dispatching is realized, for example in such a manner that, in addition to the number of its own dispatcher group, the memory of mobile station also comprises the before-programmed number of the night dispatcher group. During night dispatching, radio phones dial the night dispatcher number. The drawback is that the users of the mobile stations must know exactly when to dial the number of their own dispatcher group, versus when is dial the number of the night dispatcher group. Also, it is then necessary to program beforehand into a radio phone the night dispatcher number, i.e. the group call number of an expanded group call, which is naturally difficult.
Another way of implementing the night dispatching or expanded group call facility is that separate dispatcher groups are interconnected in the mobile communication system so that it is always possible to dial the same dispatcher group number. Small dispatcher groups thereby still operate within their own areas.
The disadvantage of this arrangement is that the radio phones, i.e. mobile stations, may then operate only within their own small dispatching area. In prior art systems, the problem has been solved by programming the numbers of all small dispatcher groups into all radio phones of the night dispatcher groups. However, this always requires advance preparations and a lot of work, as well as expert programming of all mobile stations. In such a case, the user of a mobile station must decide himself or herself in which group call he or she wants to participate within each dispatching area, i.e. group call area. The disadvantage of the arrangement is that forming and changing the night dispatcher groups and areas is difficult, since the programming must be performed in each radio phone.
Another drawback is that when this arrangement is used, it is necessary to reserve several radio channels from each radio cell, i.e. base station, for the expanded group call to be established, which necessarily wastes radio resources both in the form of channels and radio units maintaining them.
The functions of call diversion, i.e. call forwarding, and immediate call diversion that are shown in FIG. 3 are generally known in telephone technology. Call forwarding refers to a function by means of which calls to a subscriber can be transferred to the exchange or to another subscriber. FIG. 3 shows a first mobile station, i.e. subscriber A, 301, an exchange 304, a second mobile station 303, and a third mobile station 302.
In normal call forwarding, subscriber A 301 performs a call set-up request 305, i.e. performs an individual call to establish a connection. Since the call forwarding facility is now activated, the exchange 304 then sets up two calls: call 1 306 between subscriber A and the exchange, and call 2 307 between subscriber C receiving the call to be transferred and the exchange. The exchange 304 connects these calls, and subscribers A and C may communicate with each other. The prior art call forwarding is reasonable in a wire network and also in individual calls in mobile communication systems. No call forwarding of group calls is known in prior art systems. If the prior art call forwarding were utilized in group calls, radio channel capacity would be wasted since two group calls must be established: the original requested group call and the new group call to which the original call is connected. If the group call groups that are to be connected to the same call are situated within the same geographical area in the mobile communication system, each group call must be provided with its own channel capacity at least partly within the same geographical areas, which in turn wastes channel capacity.
A mobile communication system implemented in the manner described above would work well if during an expanded group call to be established in a dispatching, for example night dispatching situation, the mobile stations would only have to communicate with the dispatchers. But if the mobile station initiating the call wants to set up a group call, i.e. if it must be able to communicate both with the dispatcher and with all other mobile stations belonging to the same group call group and situated in the area, the above-described arrangement is not sufficient.